1. Field of the Invention
The present invention relates to an automobile wheel cast of light alloy.
2. Description of the Prior Art
Light alloy cast wheels used for automobiles have a configuration typically shown in FIG. 4 and are molded as shown in FIG. 5.
As illustrated in FIG. 4, the light alloy cast wheel 110 includes a disk portion 112 and a rim portion 114 constructed integral with the disk portion 112. The wheel 110 is mounted to an axle shaft of an automobile at the disk portion 112 and supports a tire at the rim portion 114. The rim portion 114 includes rim flanges 116 and 118 at opposite ends thereof, which radially extend and are curled at their radially outer portions thereof. The rim portion 114 further includes bead seat portion 120 and 122 which are connected to the rim flanges 116 and 118, respectively, and extend in an axial direction. Large loads act on the rim flanges 116 and 118 and the bead seat portions 120 and 122. Rim flanges 116 and 118 receive an axially acting pressure load from the tire, and the bead seat portions 120 and 122 receive a reaction force from the ground through the tire.
In a low pressure casting of the light alloy wheel, as illustrated in FIG. 5, a molten low alloy metal 150 is biased by pressure into a cavity formed between a first mold 152 and a second mold 154. The molten metal 150 enters the cavity at a central portion of the cavity which corresponds to a boss portion of the disk portion 112. Therefore, the molten metal 150 initially fills a portion of the cavity corresponding to the disk portion 112 and then fills a portion of the cavity corresponding to the rim portion 114. After a predetermined period of time has elapsed and the metal poured into the cavity of the molds has coagulated, the molds are opened so that the wheel can be removed from the molds.
However, there is a problem in the above-described conventional casting. More particularly, the rim flange 116 located farther from the disk portion 112 is constructed so as to have a larger thickness than the bead seat portion 120 connected to the rim flange 116 because the rim flange 116 must endure a large side load from the tire. As a result, when the molten metal in the cavity coagulates, a portion of the molten metal filling the bead seat portion 120 will coagulate before a portion of the molten metal filling the rim flange 116 which is farther from the disk portion 112. Therefore, when the flange filling molten metal coagulates after the bead seat portion filling molten metal, movement of molten metal from the boss portion to the rim flange 116 through the bead seat portion 120 stops, and blow holes will occur in the flange filling molten metal as it is cooled. A considerably many blow holes will occur because the rim flange 116 has a relatively large thickness and shrinks by a relatively large amount. Since the rim flange 116 must have a sufficient strength to endure the side pressure load from the tire, blow holes are undesirable.